The present invention relates to emission control apparatus for an internal combustion engine, and, in particular, to a device for determining the exhaust gas recirculation rate of an internal combustion engine.
One known way of lowering NOx emissions from spark-ignition or diesel engines is what is known as exhaust gas recirculation (EGR). In particular, the temperature-sensitive NOx emission is reduced by admixing exhaust gases to the fresh air prior to combustion. Usually, exhaust gas is added to an intake line from an exhaust pipe of the internal combustion engine via an exhaust gas recirculation line. The recirculation rate of the exhaust gas, i.e., the quantity of the exhaust gas stream which is admixed with the fresh air stream in the intake line in relation to the combustion air quantity, is controlled by a controller unit in adaptation to set values predetermined as a function of the particular operating point. The extent to which the exhaust gas recirculation rate can be increased with a view to further reducing the NOx emissions is subject to different limits, for example as a function of the operating point, specifically by the particulate emissions, which increase above certain recirculation rates, the fuel consumption and by the deterioration in the smooth running of the internal combustion engine.
Controlling the exhaust gas recirculation rate requires the ongoing recording of the prevailing recirculation conditions during operation of the internal combustion engine.
European Patent Application No. 057 4 614 A1 discloses a method for controlling the exhaust gas recirculation rate which, to determine the actual value of the recirculated exhaust gas mass flow, measures the pressure drop at a venturi nozzle in the exhaust gas recirculation passage and determines the through-flow from the pressure difference.
German Patent Application DE 100 070 10 A1 discloses a sensor unit for determining the exhaust gas recirculation rate of an internal combustion engine. The sensor unit is acted on by the exhaust gas atmosphere inside the exhaust gas recirculation line, on one side, and by the atmosphere comprising exhaust gas and fresh air which is present within the intake line of the internal combustion engine downstream of the location where the exhaust gas recirculation line opens out, on the other side. The two gas atmospheres acting on the sensor unit being kept separate from one another.
German Patent DE 197 34 494 C1 discloses a method for operating an internal combustion engine with recirculation of exhaust gases from an exhaust pipe into an intake line through an exhaust gas recirculation passage. To allow the maximum lowering of emissions by exhaust gas recirculation with optimally accurate recording and control of the exhaust gas recirculation rates, it is provided that the prevailing oxygen content in the gas streams upstream and downstream of the location where the exhaust gas recirculation passage opens out into the intake line be measured by means of sensors and that the controller unit accurately determines the recirculation rate from the measurement results.
To determine the EGR rate, it is necessary to respond as quickly as possible to changes in concentration at a sensor, in order, by means of an engine control, to control or limit the emission of pollutants.
In a motor vehicle, conventional positioning of a sensor on the wall of the induction manifold represents a problem insofar as a sensor positioned in this way is unable to react optimally to rapid changes in, for example, an NOx concentration, since the flow velocity of the gas that is to be detected drops towards zero at the wall of the manifold. For this reason, inter alia, a change in the gas composition will affect the sensor signal with a time delay, since the new or altered gas composition has to flow over the entire sensor surface.
It is an object of the present invention to determine an EGR rate in as simple a way as possible, but with the maximum possible accuracy.